Ultrasonic transducers in combination with cross-correlation techniques are used to determine the flow rate of a gas or liquid flowing in a conduit. The cross-correlation technique has an advantage in low pressure gas flow measurement and clamp-on high temperature liquid flow measurement because the transmitted signal or transmitted signal to noise ratio need not be as high as required for a transit time meter. In this measurement technology, two axially spaced ultrasonic transmitters are clamped to one side of the conduit and two corresponding ultrasonic receivers are clamped to an opposite side of the conduit. A “tag” in the flow in the conduit modulates frequency and amplitude of the ultrasonic signals launched by the transmitters and the resulting signals modulations received by the receivers are cross-correlated to determine the travel time of the tag from one transmitter/receiver pair path to the other. Since the path length is known, the flow rate of the tag and thus the flow rate of the flow in the conduit can be determined. See U.S. Pat. No. 6,293,156, column 2, lines 1-17. This patent is incorporated herein by this reference. The focus of the '156 patent disclosure is a method of reducing cross talk (noise) in such a system whereby the transmitters are mounted so as to create a shadow zone and the receivers are positioned in the shadow zone.
Usually, care is taken to precisely align the receiver with the highest intensity region (i.e., the center axis) of the ultrasonic beam launched by the transmitter so signals of sufficient amplitude are detected by the receiver so that the modulations of the transmission can be obtained (modulation is the signal in this technique).
When the flow rate is low and the flow turbulence is utilized as the tag which may be the case in a wide variety of industry applications such as measuring the high temperature flow rate of feed water in a nuclear power plant, the modulation or “signal” produced by the tag is generally very weak. In such situations, averaging over a long period of time is required, the response time is slow, and the accuracy of the flow rate measurements can suffer considering that the flow rate can change. Therefore, there is a need to use a correlation technique to measure flow with low transmitted signal. There is also a need to have a cross correlation meter able to measure low flow.